T H E S T A T E O F S IL U R IA N S T R A T IG R A P H Y

by

C .H . H o l la n d

T h is sh o r t a r t ic le r e v ie w s r e c e n t d e c is io n s tak en bv th e IU G S S ub co m m issio n on Stratigraphy, 丈h e procedures by w h ic h r e c o m m e n d a tio n s a r e m ade and som e o厂the challenges sti11 加cinq workers on t八e S ilu r ia n .

introdu ction The geological sciences dem and close inter n ational cooper- For stages w ithin these tw o series, the Subcom m ission ation as w ell as a rigorous and clearly understood language of agreed to use the "Sheinw oodian" and "H orrierian" stages w ords. N ow here is this m ore keenly felt than in stratigraphy for the W enlock. These again had been defined by Bassett w here, despite the luxury of long philosophical discussions on e t al. (1975). The boundary stratotype for the base of the rocks versus tim e, m any feel a sense of urgency about fo rm er w a s to be the sam e a s th a t for the W e n loc k S erie s. attem pts to achieve a standard global . A boundary stratotype for the base of the H om erian The now w ell-know n pioneer w ork of the international com - w as accepted w ithin the W enlock area. m ittee on the Silu ria n- D evonian Boundary took 12 to In the case of the Series, H olland et al. (1963) had com plete, though th is w a s a p io n e e r in g e x e rc ise . C o r re l- nt in n s abo u t this le v e l w e r e a ls o fo u n d t o h a v e b e e n in su c h previously defined four stages, but the first and second of these w ere difficult to separate outside the W elsh Borderland grievous error that m uch readjustm ent and eventualc o 川 p r o - and the third (Leintw ardinian) stage corresponded to only one m ise w e re n eed e d before agreem ent could be re a ch e d graptolite biozone, w idespread though that is. A ccordingly, (Boucek, of al., 1966; H olland, 1965; M artinsson, 1977). N ow the Subcom m ission decided that only tw o stages w ere approp- the lU G S C om m ission on Stratigraphy, thanks especially to riate for the L udlow and, as in the case of the W enlock, these the vision and initiative of past C haIrm an A cadem ician V.V . m enner of the Soviet U nion, acts as an um brella to a w hole correspond broadly to tw o brachiopod faunas as w ell as being tied m ore precisely to the graptolite biozonal sequence (Fig. list of Subcom m issions for the various geolog ica l system s and 2). M any other groups are of course recognized as actually or of w orking groups concerned w ith settling the boundaries. potentially useful in correlation. The Subcom m ission on Silurian Stratigraphy w as constituted T hose concerned m ost directly w ith w ork in the Ludlow area from an a生垫签 body at a m eeting held in the U niversity of w ere asked to suggest nam es for these tw o new stages and Birm ingham , U .K ., in 1974. A t a form al m eeting of the the Subcom m ission approved their choice of "G orstian" and Subcom m ission held during the International G eological C on- "" (H olland et al. 1980). The boundary stratotype gress in Sydney, A ustralia, in A ugust 1976, it w as agreed to for the base of the G orstian Stage is the sam e as that for the proceed w ith an eight- program designed to solve the b ase o f the L u d lo w Se ries . F o r th e ba se of the L ud fo rd ian m ajor problem s of Silurian chronostratigraphy. W ork contin- Stage, also sited within the Ludlow area (Fig. I），the decision ued using questionnaires and e ventually there w as alm ost is to use the boundary proposed by Holland et al．(1963) for unanim ous support for a com prehensive field m eeting in the base of their Leintw ardinian Stage. Britain in 1979. This m eeting ranged frorn South W ales, through the W elsh Borderland, N orth W ales, and the L ake T hese decisions w ere presented to the C om m ission on Stra- D istrict, to M offat in the Southern U plands of Scotland. tigraphy at its m eeting in Paris in A ugust 1980. Professor Various form al m eetings of the Subcom m ission w ere held, A nders M artinsson and D r. M .G . Bassett, respectively C hair- including shared m eetings w ith the W orking G roup on the m an and Secretary of the C om m ission, then circulated docu- O rdovician-Silurian Boundary since these tw o groups had a m entation and arranged a postal vote by all m em bers of the num ber of m em bers in com m on. The type areas of the C om m ission. The decisions already outlined above w ere I-landovery, W enlock, and Lud low series w ere exam ined, as ratified w ithout dissent (see M artinsson et al., 1981). w as C harles Lapw orth's fam ous locality for the O rdovician- Siluria n tra nsitio n at D o bb 's L in n n ea r M o ffa t. C ocks, Toghill, and Z iegler (1970) had already divided the Llandovery Series in to four stages, equated respectively w ith T he Su bc o m m issio n ha s 16 T itu la r M e m be rs an d a co n - the m uch used lettered and num bered divisions A l一A 4, B I-B 3, siderably larger num ber of C orresponding M em bers. T hose C l-C 3. and C }-C 6. These w ere given geographical nam es present at individual m eetings took part in discussions and based on localities w ithin the type area near Llandovery in decision m aking, but form al proposals w ere Put to postal vote South W ales. As the low est A l division at Liandovery proved by the Titular M em bers. D ecisions achieved by a substantial to be unfossiliferous and of uncertain extent, these authors m a)ority w ere : chose Dobb's L inn for the definition of the base of their first, T he sec o nd a nd th ird o f the fo u r series into w h ic h the R huddanian Stage. The basal boundaries of the rem aining Silurian System is to be divided w ere to be called ”W enlock Idw ian, Fronian, and stages w ere referred to local- Series" and "Ludlow Series", and boundary stratotypes for ities w ithin the Llandovery area. U nfortunately, detailed the ba se o f e a ch o f th ese w e re c h osen in the W elsh faunal c hanges t hrough c ontinuous boundary stratotype Borderland, w here fully described standard areas provide a sections w ith accurately defined m arker po ints w ere not setting for these sections. These are indicated in Figure I available and indeed there w as m uch discussion by Subcom - and details of the decisions w ere reported in 兰ethaia m ission m em bers on the difficulties of discrim inating be- (H olland 1980). F ull descriptions of the areas and sections tw een at least som e of these stages elsew here in the w orld. are to be found in Bassett et al. (1975) for the W enlock, A Subcom m ission w orking group is now looking carefully at and H olland et al. (1963) for the Ludlow . The sections are the so-called "northern area" of the Liandovery district, now cared for by the N ature C onservancy. w here m ore graptolites m ay be available. But w hatever the

EPISO D ES, V ol. 1982, N o. 3. 2 1 N ‘ ． ｜ ｜ k5 10 D obb's Linn and A nticosti for this purpose. They are con- －t ｜ cerned also w ith the difficult question of the biostratigraphi- ｜ ｜ cal horizon best chosen for the boundary. ｜ e a ｜ The fourth. post-Ludlow pre-G edinnian series of the Silurian System is the division w hich w as recognized as a result of the w ork of the Silurian-D evonian Boundary C om m ittee (M ar- tinsson, 1977). E rrors in previous correlations had been such that the rocks deposited in this interval had actually been "lost", m ainly because of the assum ption that graptoloids m ust in som e (m ystical) sense be absent fro m post-Silurian 夕’健 2 strata. T he top of the fourth series is already defined by the now agreed base of the at the base of the M ono- 三 M U C H ：· graptus uniform is Biozone, as recognized in the boundary W E N LO C K stratotype section at K lonk in the Barrandian area of C zech- oslovakia. A lthough there has been considerable discussion

C H U R C H and three subm issions have already been circulated, the S T R E T T O N A boundary stratotype for the base of the fourth series, and indeed its form al nam e, are still to be decided b y the S u b c o m m is s io n . The three candidates for the series (Fig. 2) are the "Downton Series", "Pridoli Series" and "Skala Series", or perhaps an alternative nam e. These w ould be represented by basal boundary stratotypes in, respectively, the W elsh Borderland, the Barrandian area, or Podolia (U .S.S.R .). T he D ow nton 飞 Series occurs in classic country w here tw o of the Silurian series are already defined and its basal division, the Ludlow Bone Bed, is fam iliar to geologists. H ere, how ever, the facies is of m arine-brackish character. Bassett et a]. (1980) have review ed the considerable biostratigraphical data avail- able, together w ith the correlation of the series across 生三兰三乞 全 3兰争 Europe and elsew here, through the use of a chain of evidence 于 二 ；干 石石 吕J 冲们 It 二二〔 拜 involving ostracodes, spores, vertebrates, and graptolites. K aljo (1978) has also detailed this w eb of correlative evi- d e n c e . 几T h e P rid oli roc ks of C ze c ho slov ak ia h a ve b e en c lassic sin ce the work of Barrande. The base of the D evonian is already defined here. T he strata are w ell know n and w ell represented －一一火in m useum collections. There are both grapto)itic and Shelly faunas. T he third potential stratotype is in the area of the 月 K E Y D n estr R ive r a nd its trib uta rie s in P od o lia w h e re rive r c liffs A N D YO U N G E R D O W N TO N IA N ［口 R O C K S and other exposures clearly display a sequence of h ighly 霆 ＜ fo ss ilif e r o u s S ilu r ia n t o lo w e s t D e v o n ia n ro c k s in m a r in e L U D LO W W E N LO C K T Y P E A R E A 吕 B facies and w ith structure of extrem e sim plicity. Brachiop ods L U D L O W T Y P E A R E A W E N LO C K and ostracodes are especially w ell represented in the Skala.

BO U N DA RY S TRA TO TY P ES The Subcom m ission is grateful to A cadem ician B.S. Sokolov L L A N D O V E R Y 巨 （I 一A ） and his colleagues w ho are arranging detailed exam ination of the Skala Series in 1983, w hen it should be possible to assess Figure 1; G eological m ap o厂the W elsh B orderland show ing th e relative m e rits o f a ll thre e a rea s un de r c on sid e ra tio n . Silurian subdivisions and standard areas 厂or the W enlock and M any m em bers have already seen the D ow nton and Barran- Lud low series. B oundary stratotype sections as sho wn in than areas, though few have previously visited Podolia. O p- F igure 2 are num bered as follows: 1. H ughZey Brook; 2. portunities are available for any necessary visits to the tw o W hitw ell C oppice; 3. P itch C oppice; 4. Sunnyh ill Q uarry. form er areas. A s yet there is no dem and for the definition of stages w ithin the fourth Silurian series, though these w ill final decision, the original 土y匹 (as distinct from standard) do ub tless be c om e the c on ce rn o f the Su bc om m issio n in futu re area m ust continue to be regarded as providing significant y e a r s . parastralotypes in Llandovery stratigraphy. O nce decisions have been m ade through the proper use of the In the m eantim e, in the sum m er of 1981 the Subcom m ission international m achinery provided by the C om m ission on Stra- had the opportunity to exam ine an exceedingly fossiliferous tigraphy and its subsidiary bodies, it is exceedingly im portant and beautifully exposed L landovery sequence on A nticosti that these should be im plem ented nationally and internation- Island, Q uebec, C anada, w here the structure is extrem ely ally. Publicity for the decisions is necessary: hence the sim ple (Barnes, 1982). In A ugust 1982, it exam ined another presentation of this interim report on the activities of the prom ising sequence in the O slo district of N orw ay and, Subcom m ission on Silurian Stratigraphy and the publication of hopefully in 1983 it w ill be possible to m ake decisions on various short statem ents of decisions already reached as boundary stratotypes for the chronostratigraphy of the first variously referred to above . o f the S ilu ria n se ries. T h e base of th e first o f th e tw o o r three stages into w hich the "L landovery (or A nticosti) Series" R e fe r e n c e s is likely to be divided m ust be that of the Silurian System , and here the W orking G roup on the O rdovician-Silurian Boun- Barnes, C .R ., 1982, O rdovician-Silurian Boundary Studied: dary is in the process of discussing the relative m erits of EPISO D E S, t982/t, p.33.

E PISO D E S, Vol. 1982, N o. 3. 2 2 C H R O N O S T R A T IG R A P H Y │ L O C A T I O│N O F │ B O U N D A R Y S T R A T O T│Y P E S │ │ │ u n ifo rm is │ tran sgredi│ens │ S ！L U R IA N │D ow nton Series ？ (d i│vision in to │ W elsh Borderland ？ │ │ S Y S T E M │ o r │ stages to c.ait │ Barrandian a rea ？ │ │ Ph'do ll Se ries ？ n│ec e ssity) │ Podolia ？ │ │ │

o r │ │ │ │ │

S ka la S│er i e s ？ │ │ │ │ 2 ││

2 │ │ Ludlow S e ries │ L u d f │o r d i a│n Stage │ Ludlow district ｛Sunnybill Quarry ｝ │

lein tw ar│d in en s is │ tu m e sce ns, ││ (=inc加iens│) │ G orstian Stage │Ludlow district ( Pitch Coppice｝ ││ │ │

n ilss on i s.l. │ │ lud en s is │ │

W e nloc k S e rie s H o│m e ria n S tag e │ W enlock district (W hitw ell Coppic│e) │ │

lu n d g ren │i │ e lle s a e │ │ S heinw oodian S tage │ W enlock district (Hughley Brook) │ │ │ │ cen trifu│g us │ cre n u la ta │ │

A nticosti Series ？ │stages (probably │ Anticosti ？ │ │ │

o r │ t h re e ) to 一b e │ L ia n d o w e r y d │is t r i c t ？│ │ Liandovery S eries ？ │defined │ Oslo d istrict ？ │ │ │

户 │ │

，│ │

F4W e 2; State of Silurian chronostratigraphy. C ritica l qraptolite biozones are also indicated.

Bassett, M .G ., C ocks, L.R .M ., H olland, C .H ., R ickards, R .B. national Union of Geological Sciences Series A , No. ，）， and W arren, P .T., 1975, T he type W enlock Series: R ep. 345 p. Inst. G eol. Sci., N o. 75八3, 19p. M artinsson, A ., Bassett, M .G . and H olland, C .H ., 19 81, R at- Bassett, M .G ., Law son, J.D . and W hite, D .E., 1982, The ification of Standard Chronostratigraphical Divisions1, and D ow nto n Se rie s a s the F o u rth Se rie s o f th e Siluria n 3 Tra T o T y p e s i o r Ih e M u r ia n 3 Y Sl e M : r- t-1J U L )r- 3 q 1 7 O LI L I F . System : Lethaia, 15, 1-74. ， 3 6 Boucek, B., H orny, R . and C hlupac, 1., 1966, Silurian V ersus D evonian: A cta N us. natI. Pragae, 22B, 49-66. C ocks, L.R .M ., T oghill, P. and Z iegler, A .M ., 1970, Stage nam es w ithin the Llandovery Series: G eol. M ag. 107, 79- 8 7 . H olland, C .H ., 1965, The Siluro-D evonian Boundary: G eol. M ag. 102, 2 13-22 1. H olland, C .H ., 1980, Silurian series and stages: decisions concerning chrono-stratigraphy: Lethaia, 13, 238. H olland, C .H ., Law son, J.D . and W alm sley, V.G ., 1963, T he Silurian rocks of the Ludlow district, : Bull. Br. 八B O U T T H E A U T H O R : M us. N at. H ist., Ser. G eol. 8, p. 93-171. Prof. C harles H epw orth H olland is Professor of G eology and H olland, C .H ., Law son, J.D ., W alm sley, V.G . and W hite, D .E., M ineralogy at the D ept. of G eology, Trinity C ollege, D ublin 1980, Ludlow stages: Lethaia 13, 268. 2, Ireland. H e is a past President of the Palaeontological K aljo, D ., 1978, T he D ow ntonian or P ridolian from the point A ssociation and past C hairm an of the Stratigraphy C om - o f v iew o f the B a ltic S ilu ria n: E e sti N S V T e ad . A k ad . m ittee of the G eological Society of London. H is research Toim . 27, G eol. Series N o. 1, 5-10. interests center m ainly on Low er Palaeozoic stratigraphy and palaeontology. Since his earlier research on Silurian rocks of M artinsson, A . (ed.), 1977, The Sil urian-D evonian Boundary : the W elsh Borderland and C entral W ales, he has exam ined Schw eizerbart'sche Verlagsbuchhandlung, Stuttgard (Inter- their counterparts in m any parts of the w orld.

EP ISO D E S, Vol. 1982, N o. 3. 2 3